In commonly owned U.S. Pat. No. 3,885,204 there has been disclosed a drive system of this nature wherein the pump unit of a hydrostatic transmission is mechanically linked with the shaft of a battery-powered d-c motor of the shunt-wound type whose speed, representing the input velocity of the transmission, can be varied by changing the energization of its field winding with the aid of a potentiometer or equivalent adjustable impedence means in circuit therewith. A setting member on the hydrostatic pump unit is connected with an accelerator pedal for actuation by the driver of a vehicle equipped with this system; the pedal also controls the potentiometer in series with the field winding of the electric shunt motor so as to vary the energization of that winding in a sense accelerating the motor with progressive depression of the pedal. In this manner, for reasons fully explained in the patent, the efficiency of the system is improved.
The increase in field current under partial load, resulting from the reduced potentiometer resistance upon retraction of the pedal, conserves the energy of the supply battery since the electric motor runs at reduced speeds under these conditions. In fact, as likewise described in the prior patent, the motor may act as a generator when the vehicle is hydrostatically braked to standstill by the release of the pedal, thereby returning some of its mechanically stored energy to the battery.
The aim of optimizing the efficiency of the system is, however, only partly achieved with the solution proposed in the prior patent. Thus, the position of the pedal (and therefore of the control member of the hydrostatic pump) is not unequivocally related to either the input velocity of the hydrostatic transmission or the torque transferred to the load from the motor of the transmission. Since the product of output velocity and torque represents the power delivered to the load, either of these factors can vary at the expense of the other under given loading conditions. Conversely, the maintenance of a certain speed requires a larger torque under high load (e.g. during uphill driving) than under low load. The change in pump speed dependent upon the pedal position, jointly with a variation in the potentiometer setting, does not invariably lead to an optimum speed ratio.